This invention relates to a thermally matched interface between a heat producing electronic device and a heat sink. More particularly, this invention relates to a thermally matched gradient interface made of materials having selected coefficients of thermal expansion and thermal conductivity, the interface being interposed between a heat producing integrated circuit and a primary heat sink on a printed circuit board.
Current, high efficiency integrated circuit devices, particularly high-speed central processing units for computers, generate substantial amounts of heat energy during operation. If this heat is not continuously removed, a device may be damaged or its operating performance significantly diminished.
A preferred method of removing excess heat from central processing units is to provide a heat sink device. In critical applications, a preferred means of heat removal is the use of a fan-assisted heat sink device. The heat sink itself is formed of a material with high thermal conductivity, such as aluminum, which readily conducts heat away from the integrated circuits and may be provided with fins to increase surface area and direct air flow. The heat sink is placed over and in contact with the integrated circuit device. Generally the heat sink is held in place mechanically by a bracket or frame. Where a bracket or frame is used, thermal grease may be interposed between the integrated circuit and the heat sink device. Alternatively, the heat sink may be bonded to the integrated circuit with solder or adhesive.
In the fan-assisted heat sinks, the fan draws cooling air into contact with the heat sink body to efficiently draw heat away from the electronic device. One such fan assisted heat sink device is described in U.S. Pat. No. 5,785,116 assigned to the assignee hereof, and incorporated herein by reference.
A problem encountered with the use of such efficient heat sinks is the wear and tear on the underlying electronics due to thermal cycling. That is, stress is created at the heat sink-integrated circuit junction when the integrated circuit heats up and cools down during use. If the integrated circuit is physically bonded to the heat sink, the integrated circuit itself, the electrical connections, and the underlying printed circuit board repeatedly expand and contract at different rates due to the differing coefficients of thermal expansion of the components. If thermal grease is used between the integrated circuit device and the heat sink, the grease may be xe2x80x9cpumped outxe2x80x9d by repeated thermal cycling. If the heat sink is simply positioned adjacent the integrated circuit, there is less than optimum contact between the integrated circuit and the heat sink, reducing the conduction of heat away from the integrated circuit.
Materials with coefficients of thermal expansion similar to those of integrated circuits may be used to form heat sinks. However, these materials generally have lower thermal conductivity than those metals which are good heat conductors. Heat sinks made from such low heat conductivity materials are consequently inefficient at heat dissipation, potentially resulting in damage to the electronics, or the requirement for heat sinks that are too big and bulky to be practical.
Accordingly, a means and method have been sought to practically and efficiently remove heat from electronic devices by means of heat sinks without inducing harmful stresses.
In accordance with the invention, an assembly comprising a heat generating electronic device and a heat sink body is provided with an intermediate, heat-matching, gradient region between them. By intermediate gradient region herein is meant the region between the underlying heat generating device and the heat sink body, which has a thermal conductivity as great or greater than the underlying electronics but lower than or equal to the thermal conductivity of the heat sink. This intermediate gradient region is made up of one or more materials having a coefficient of thermal expansion substantially matching the coefficient of thermal expansion of the heat generating electronic device, and a thermal conductivity greater than the thermal conductivity of the heat-generating device but less than the thermal conductivity of the heat sink. Provision of this intermediate region substantially reduces stress on the electronic device caused by thermal cycling.
In another embodiment, a specially adapted intermediate gradient region is interposed between a heat generating component and a heat sink. The intermediate region has a first base portion with a coefficient of thermal expansion substantially matching the coefficient of thermal expansion of an underlying, heat generating component (such as a central processing unit for a computer), but a higher thermal conductivity. The intermediate gradient region further has a second portion, integral with the first base portion, that serves as the principal heat dissipating element of the xe2x80x9chybridxe2x80x9d intermediate gradient region. This second portion has a higher thermal conductivity than either the base or underlying electronic component. Accordingly, thermal cycling stresses are absorbed by the overlying intermediate region and heat sink body, rather than the underlying electronics or circuit board, preventing damage to the electronics.